Method of forming a barrier layer arrangement for conductive layers on silicon substrates

ABSTRACT

A process is disclosed of producing on a crystalline silicon substrate a barrier layer triad capable of protecting a rare earth alkaline earth copper oxide conductive coating from direct interaction with the substrate. A silica layer of at least 2000 Å in thickness is deposited on the silicon substrate, and followed by deposition on the silica layer of a Group 4 heavy metal to form a layer having a thickness in the range of from 1500 to 3000 Å. Heating the layers in the absence of a reactive atmosphere to permit oxygen migration from the silica layer forms a barrier layer triad consisting of a silica first triad layer located adjacent the silicon substrate, a heavy Group 4 metal oxide third triad layer remote from the silicon substrate, and a Group 4 heavy metal silicide second triad layer interposed between the first and third triad layers.

This is division of copending application Ser. No. 153,699, filed Feb.8, 1988, now issued as U.S. Pat. No. 4,908,348.

FIELD OF THE INVENTION

The present invention relates to electrical circuit elements and toprocesses for their preparation.

BACKGROUND OF THE INVENTION

The term "superconductivity" is applied to the phenomenon ofimmeasurably low electrical resistance exhibited by materials. Untilrecently superconductivity had been reproducibly demonstrated only attemperatures near absolute zero. As a material capable of exhibitingsuperconductivity is cooled, a temperature is reached at whichresistivity decreases (conductivity increases) markedly as a function offurther decrease in temperature. This is referred to as thesuperconducting transition temperature or, in the context ofsuperconductivity investigations, simply as the critical temperature(T_(c)). T_(c) provides a conveniently identified and generally acceptedreference point for marking the onset of superconductivity and providingtemperature rankings of superconductivity in differing materials.

It has been recently recognized that certain rare earth alkaline earthcopper oxides exhibit superconducting transition temperatures well inexcess of the highest previously known metal oxide T_(c), a 13.7° K.T_(c) reported for lithium titanium oxide. These rare earth alkalineearth copper oxides also exhibit superconducting transition temperatureswell in excess of the highest previously accepted reproducible T_(c),23.3° K. for the metal Nb₃ Ge.

Recent discoveries of higher superconducting transition temperatures inrare earth alkaline earth copper oxides are reported in the followingpublications:

P-1 J. G. Bednorz and K. A. Muller, "Possible High T_(c)Superconductivity in the Ba-La-Cu-O System", Z. Phys. B.--CondensedMatter, Vol. 64, pp. 189-193 (1986) revealed that polycrystallinecompositions of the formula Ba_(x) La_(5-x) Cu₅ O₅(3-y), where x=1 and0.75 and y>0 exhibited superconducting transition temperatures in the30° K. range.

P-2 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and Y. Q.Wang, "Evidence for Superconductivity above 40 K. in the La-Ba-Cu-OCompound System", Physical Review Letters, Vol. 53, No. 4, pp. 405-407,Jan. 1987, reported increasing T_(c) to 40.2° K. at a pressure of 13kbar. At the end of this article it is stated that M. K. Wu increasedT_(c) to 42° K. at ambient pressure by replacing Ba with Sr.

P-3 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, and Z. J. Huang,"Superconductivity at 52.5 K. in the Lanthanum-Barium-Copper-OxideSystem", Science Reports, Vol. 235, pp. 567-569, Jan. 1987, a T_(c) of52.5° K. for (La₀.9 Ba₀.1)₂ CuO_(4-y) at high pressures.

P-4 R. J. Cava, R. B. vanDover, B. Batlog, and E. A. Rietman, "BulkSuperconductivity at 36 K. in La₁.8 Sr₀.2 CuO₄ ", Physical ReviewLetters, Vol. 58, No. 4, pp. 408-410, Jan. 1987, reported resistivityand magnetic susceptibility measurements in La_(2-x) Sr_(x) CuO₄, with aT_(c) at 36.2° K. when x=0.2.

P-5 J. M. Tarascon, L. H. Greene, W. R. McKinnon, G. W. Hull, and T. H.Geballe, "Superconductivity at 40 K. in the Oxygen-Defect PerovskitesLa_(2-x) Sr_(x) CuO_(4-y) ", Science Reports, Vol. 235, pp. 1373-1376,Mar. 13, 1987, reported title compounds (0.05≦x≦1.1) with a maximumT_(c) of 39.3° K.

P-6 M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao,Z. J. Huang, Y. Q. Wang, and C. W. Chu, "Superconductivity at 93 K. in aNew Mixed-Phase Y-Ba-Cu-O Compound System at Ambient Pressure", PhysicalReview Letters, Vol. 58, No. 9, pp. 908-910, Mar. 2, 1987, reportedstable and reproducible superconducting transition temperatures between80° and 93° K. at ambient pressures for materials genericallyrepresented by the formula (L_(1-x) M_(x))_(a) A_(b) D_(y), where L=Y,M=Ba, A=Cu, D=O, x=0.4, a=2, b=1, and y≦4.

The experimental details provided in publications P-1 through P-6indicate that the rare earth alkaline earth copper oxides prepared andinvestigated were in the form of cylindrical pellets produced by formingan intermediate oxide by firing, grinding or otherwise pulverizing theintermediate oxide, compressing the particulate intermediate oxideformed into cylindrical pellets, and then sintering to produce apolycrystalline pellet. While cylindrical pellets are convenientarticles for cooling and applying resistance measuring electrodes, boththe pellets and their preparation procedure offer significantdisadvantages to producing useful electrically conductive articles,particularly articles which exhibit high conductivity below ambienttemperature--e.g., superconducting articles. First, the step of grindingor pulverizing the intermediate oxide on a commercial scale prior tosintering is both time and energy consuming and inherently susceptibleto material degradation due to physical stress on the material itself,erosion of grinding machinery metal, and handling. Second, electricallyconductive articles rarely take the form of pellets. Electricallyconductive articles commonly include either thin or thick films formingconductive pathways on substrates, such as insulative and semiconductivesubstrates--e.g., printed and integrated circuits.

CROSS-REFERENCE TO RELATED FILING

Mir, Agostinelli, Peterson, Paz-Pujalt, Higberg, and Rajeswaran U.S.Ser. No. 046,593, filed May 4, 1987, titled CONDUCTIVE ARTICLES ANDPROCESSES FOR THEIR PREPARATION, now U.S. Pat. No. 4,880,770, commonlyassigned, discloses articles in which an electrically conductive layeron a substrate exhibits a superconducting transition temperature inexcess of 30° K. Conductive layers are disclosed comprised of acrystalline rare earth alkaline earth copper oxide. Processes ofpreparing these articles are disclosed in which a mixed metal oxideprecursor is coated in solution and subsequently heated to its thermaldecomposition temperature to create an amorphous mixed metal oxidelayer. The amorphous layer is then heated to its crystallizationtemperature. Thin electrically conductive films are formed.

Strom, Carnall, Ferranti, and Mir U.S. Ser. No. 068,391, filed July 1,1987, titled CONDUCTIVE THICK FILMS AND PROCESS FOR FILM PREPARATION,now U.S. Pat. No. 4,908,346 commonly assigned, discloses circuitelements comprising an insulative substrate and means for providing aconductive path between at least two locations on the substrateincluding a thick film conductor which is comprised of a crystallinerare earth alkaline earth copper oxide layer having a thickness of atleast 5 μm. The thick film conductor is formed by coating a conductorprecursor on the insulative substrate and converting the conductorprecursor to an electrical conductor. The conductor precursor is coatedin the form of particles of metal-ligand compounds of each of rareearth, alkaline earth, and copper containing at least one thermallyvolatilizable ligand. The coated conductor precursor is heated in thepresence of oxygen to form an intermediate coating on the substrate. Theintermediate coating is converted to a crystalline rare earth alkalineearth copper oxide electrical conductor.

In attempting to form an electrically conductive, particularlysuperconductive, rare earth alkaline earth copper oxide layer on asubstrate a difficulty that has been encountered is migration ofsubstrate and copper containing oxide layer elements upon heating to thehigh temperatures required for crystallization, typically in the rangeof from 900° to 1100° C. Migration alters the composition of the coppercontaining oxide layer and interferes with formation of the crystalstructures required for best conductivity results. While the difficultyof substrate contamination of the copper containing oxide layer can beameliorated to a degree by increasing its thickness, the choice ofsubstrates which produce better results in terms of copper containingoxide layer conductivity has remained restricted, particularly informing thin (<5 μm) film thicknesses.

Agostinelli, Mir, Paz-Pujalt, Lelental, and Nicholas U.S. Ser. No.85,047, filed Aug. 13, 1987, titled BARRIER LAYER CONTAINING CONDUCTIVEARTICLES, now abandoned in favor of U.S. Ser. No. 330,409, filed Mar.30, 1989, discloses a circuit element comprised of a substrate and anelectrically conductive layer located on the substrate. The circuitelement is characterized in that the electrically conductive layer iscomprised of a crystalline rare earth alkaline earth copper oxide, thesubstrate is formed of a material which increases the electricalresistance of the conductive layer when in contact with the rare earthalkaline earth copper oxide during its crystallization to anelectrically conductive form, and a barrier layer is interposed betweenthe electrically conductive layer and the substrate. The barrier layercontains a metal, in its elemental form or in the form of an oxide orsilicide, chosen from the group consisting of magnesium, a group IVAmetal, or a platinum group metal.

Agostinelli et al., additionally discloses a process of forming acircuit element including coating a conductor precursor on a substrateand converting the conductor precursor to an electrical conductor. Theprocess is characterized by the steps of choosing, as the conductorprecursor, metal-ligand compounds of each of rare earth, alkaline earth,and copper containing at least one thermally volatilizable ligand;heating the precursor metal-ligand compounds in the presence of oxygento produce a crystalline rare earth alkaline earth copper oxideelectrically conductive layer; choosing as the substrate a materialwhich increases the electrical resistance of the conductive layer whenin contact with the rare earth alkaline earth copper oxide during itscrystallization to an electrically conductive form; and prior to coatingthe conductor precursor on the support forming on the substrate abarrier layer. The barrier layer contains a metal, in its elemental formor in the form of an oxide or silicide, chosen from the group consistingof magnesium, a group IVA metal, or a platinum group metal.

SUMMARY OF THE INVENTION

The present invention is directed to protecting silicon substrates andconductive layers coated thereon from mutual contamination.Specifically, the invention is directed to improving the electricalconduction characteristics of a rare earth alkaline earth copper oxidelayer when coated on a silicon substrate while at the same timeprotecting the silicon substrate from copper contamination. Thisobjective is achieved by a particular arrangement of layers interposedbetween the rare earth alkaline earth copper oxide layer and the siliconsubstrate and by a process for producing this layer arrangement. Theinvention permits the formation on silicon substrates of rare earthalkaine earth copper oxide layers which exhibit higher criticaltemperatures and superconductivity at higher temperatures. The inventionfurther permits the rare earth alkaline earth copper oxide conductivelayer to carry higher current densities in its superconducting state.

In one aspect this invention is directed to a circuit element comprisedof a silicon substrate and a conductive layer located on the substrate.The conductive layer consists essentially of a rare earth alkaline earthcopper oxide. A barrier layer triad is interposed between the siliconsubstrate and the conductive layer comprised of a first triad layerlocated adjacent the silicon substrate consisting essentially of silica,a third triad layer remote from the silicon substrate consistingessentially of at least one Group 4 heavy metal oxide, and a secondtriad layer interposed between the first and third triad layersconsisting essentially of a mixture of silica and at least one Group 4heavy metal oxide.

In another aspect of the invention, the circuit element is furthercharacterized in that at least 45 percent by volume of the rare earthalkaline earth copper oxide of the conductive layer is in an R₁ A₂ C₃crystalline phase and silver is located at the interface of the thirdtriad layer and the conductive layer in an amount sufficient to directcrystal orientation of the R₁ A₂ C₃ crystalline phase.

In an additional aspect, the invention is directed to a process ofproducing on a silicon substrate a barrier layer triad comprising (1)forming a silica layer of at least 2000 Å in thickness on the siliconsubstate, (2) depositing on the silica a layer having a thickness in therange of from 1000 to 5000 Å of at least one Group 4 heavy metal, and(3) heating the layers in the absence of a reactive atmospheresufficiently to permit oxygen migration from the silica layer, therebyforming a barrier layer triad consisting of a first triad layer locatedadjacent the silicon substrate consisting essentially of silica, a thirdtriad layer remote from the silicon substrate consisting essentially ofat least one Group 4 heavy metal oxide, and a second triad layerinterposed between the first and third triad layers consistingessentially of at least one Group 4 heavy metal silicide.

In an additional aspect of the invention, the process described above isfurther characterized in that the barrier layer triad is subsequentlyheated in the presence of oxygen to convert the second triad layer to amixture of silica and at least one Group 4 heavy metal oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the invention can be better appreciated byreference to the following detailed description of preferred embodimentsconsidered in conjunction with the drawings, in which

FIG. 1 is a schematic diagram showing process steps and articlesproduced thereby;

FIG. 2 is a schematic diagram of a portion of a preferred thin filmprocess;

FIG. 3 is a schematic diagram of a pattern producing sequence of processsteps; and

FIG. 4 is a schematic diagram of a portion of a preferred thick filmprocess.

DESCRIPTION PREFERRED EMBODIMENTS

The present invention has as its purpose to make available electricalcircuit elements containing a silicon substrate supporting a conductiverare earth alkaline earth copper oxide layer. The silicon substrate andthe rare earth alkaline earth copper oxide layer are protected frommutual contamination by an interposed barrier layer triad.

The silicon substrates contemplated by this invention include anyconventional monocrystalline or polycrystalline substrate. Amorphoussilicon can also be employed as a substrate, but it will be converted toa polycrystalline form by the temperatures required for forming the rareearth alkaline earth copper oxide conductive layer thereon. It isspecifically contemplated to employ as substrates silicon semiconductiveelements, particularly silicon integrated circuits.

To protect the silicon substrate from contamination by the rare earthalkaline earth copper oxide forming the overlying conductive layer,particularly to protect against copper contamination, and to protect theconductive layer from silicon contamination which will reduce itsdesirable electrical conduction properties, particularly siliconcontamination which will lower the Tc of the conductive layer ordecrease the maximum temperature at which superconductivity is observed,a barrier layer triad is formed on the silicon substrate surface beforethe conductive layer is formed.

The first step of the process is to form on the silicon substrate asilica layer of at least 2000 Å in thickness. The silica layer ispreferably at least 5000 Å in thickness. Any convenient thickness of thesilica layer can be formed beyond the minimum thickness required. Silicalayer formation on silicon substrates can be achieved by any convenientconventional means and forms no part of this invention. The silica layeris in most instances conveniently formed by oxidation of the siliconsubstrate surface to form a grown silicon oxide layer. Alternatively,the silica layer can be deposited.

Following formation of the silica layer at least one Group 4 heavy metalis deposited on the silica layer. The term "Group 4 heavy metal" isherein employed to indicate the elements zirconium and hafnium, whichoccupy Group 4 of the periodic table of elements, as adopted by theAmerican Chemical Society. Group 4 elements are also referred to asGroup IVA elements using the IUPAC notation system for the periodictable of elements. The Group 4 heavy metal or metals are deposited toform a layer which is from about 1000 to 5000 Å in thickness. At lowerlayer thicknesses barrier effectiveness can be degraded by inadequateGroup 4 heavy metal while at greater layer thicknesses physical stressesin the layer are increased, which can lead to physical defects. TheGroup 4 heavy metal layer can be deposited on the silica layer by anyconvenient conventional technique compatible with achieving the desiredlayer thickneses. Techniques such as vacuum vapor deposition, chemicalvapor deposition, metal organic vapor deposition, and sputtering areadequate to achieved the desired layer arrangement.

The next step is to convert the silica and Group 4 heavy metal layersinto a barrier layer triad. The first step toward achieving thisobjective is to heat the two initially formed layers in the absence of areactive atmosphere to a temperature at which oxygen migration canoccur. Heating is best accomplished in a vacuum or in an inert gasatmosphere, such as an argon atmosphere. The objective is to driveoxygen out of the silica layer into the Group 4 heavy metal layer. Thisforms Group 4 heavy metal oxides in the Group 4 heavy metal layer at alocation remote from the underlying silica layer. At the same time theGroup 4 heavy metal reacts with the silica layer at its interface toform Group 4 heavy metal silicides.

The result is to form a barrier layer triad comprised of a first triadlayer consisting essentially of silica, a third triad layer consistingessentially of at least one Group 4 heavy metal oxide, and a secondtriad layer of the barrier triad interposed between the first and secondtriad layers consisting essentially of at least one Group 4 heavy metalsilicide.

The time and temperature of heating required to produce the necessaryoxygen migration are interrelated. Oxygen migration can be achieved atlower temperatures and longer heating times or at higher temperaturesand shorter heating times. Generally temperatures of at least 700° C.are required to produce significant oxygen migration, even when heatingtimes are extended for 2 or more hours. On the other hand, it is notgenerally contemplated to employ heating temperatures in excess of about1200° C. It is preferred to employ heating temperatures in the range offrom about 750° C. to 1000° C., with optimum heating times being in therange of from about 800° C. to 900° C. While adequate oxygen migrationcan occur by rapid thermal annealing at near maximum tempertures in timeperiods as low as about 1 minute, generally preferred heating times arefrom about 30 to 60 minutes.

Although the second triad layer as initially formed of Group 4 heavymetal silicide, this layer in the final conductive element is a mixtureof silica and Group 4 heavy metal oxide. This transformation resultsfrom subsequent heating in the presence of oxygen. This can be achievedby a separate heating step similar to that described above, but withoxygen being present. However, in most instances no additional heatingstep is introduced to convert the second triad layer from a silicidelayer to an oxide layer, since this is achieved by heating steps to besubsequently described employed in creating at conductive crystallinerare earth alkaline earth copper oxide layer.

From the foregoing description it can be appreciated that the siliconsubstrate and the barrier layer triad together form the followingsequence:

    ______________________________________                                        Third Triad Layer                                                             Second Triad Layer                                                            First Triad Layer                                                             Silicon Substrate                                                             ______________________________________                                    

In completed conductive elements of this invention the first triad layerhas a thickness of at least about 1000 Å. The maximum thickness of thislayer is not critical and can range above 1 μm, if desired. The secondand third barrier triad layers also each preferably have thicknesses ofat least about 1000 Å, but typically each have thicknesses of 5000 Å orless.

Formation of the conductive rare earth akaline earth copper oxide layercan be undertaken directly on the third triad layer, if desired. It ispreferred, however, first to deposit silver on the third triad layer.The silver deposit has the unexpected and advantageous effect ofimproving the conductive properties of rare earth alkaline earth copperoxide layers in the the R₁ A₂ C₃ crystalline form, described below. Thesilver facilitates formation of the desired crystal structure in theoverlying conductive layer and further acts the orient the crystals inthe overlying conductive layer to permit higher current densities of therare earth alkaline earth copper oxide layers at superconductingtemperatures.

The silver can be deposited by any convenient conventional technique.The silver is preferably deposited by techniques, such as vacuum vapordeposition, metal organic compound decomposition, chemical vapordeposition, or sputtering. The silver as initially deposited preferablyis provided in a layer thickness of from about 500 to 2000 Å.Microscopic examination of silver in the completed conductive elementsreveals the silver to be present as a crystalline layer, which for lowsilver coverages can be discontinuous. Thus, in the final device it isnot the layer thickness that is important, but rather the presence ofsufficient silver to orient crystal growth of the rare earth alkalineearth copper oxide layer. Silver coverages are adequate for this purposethat provide at least 50 micrograms (μg) of silver per squarecentimeter, preferably 50 to 200 μg/cm². The maximum silver coverage isnot critical and is limited only by convenience and economicconsiderations.

The term "rare earth alkaline earth copper oxide" refers to acomposition of matter containing at least one rare earth element, atleast one alkaline earth element, copper, and oxygen. The term "rareearth" is employed to designate yttrium and lanthanides--i.e., elementsof the lanthanide series. Lanthanum, samarium, europium, gadolinium,dysprosium, holmium, erbium, and ytterbium are particularly preferredlanthanides. The term "alkaline earth" indicates elements of Group 2 ofthe periodic table of elements as adopted by the American ChemicalSociety. Calcium, strontium and barium are preferred alkaline earthelements for the practice of this invention.

In keeping with the established practice in the ceramics are ofshortening lengthy chemical names of mixed metal oxides by substitutingacronyms based on the first letters of the metals present, the term"RAC" is hereinafter employed to indicate generically rare earthalkaline earth copper oxides. When it is intended to designatespecifically a lanthanide or yttrium as the rare earth component, L orY, respectively, is substituted for R; and when it is intended todesignate specifically strontium or barium as the alkaline earthcomponent, S or B, respectively, is substituted for A.

A preferred process for preparing an electrical circuit elementaccording to the present invention is schematically illustrated inFIG. 1. Step A collects together the steps described above for formingthe barrier layer triad, either with a silicide or a mixed metal oxidein the second layer. The resulting coated article as schematically shownconsists of silicon substrate 3 and barrier layer triad 5.

In Step B the optional, but highly preferred additional step ofdepositing silver is performed. The barrier clad substrate 7 resultingconsists of silicon substrate 3 and the barrier layer triad and silverlayer, schematically illustrated as composite barrier layer 9.

In Step C of the preparation process, onto the composite barrier layeris coated a composition consisting essentially of RAC precursors(metal-ligand compounds of each of rare earth, alkaline earth, andcopper) containing at least one thermally volatilizable ligand. Theresulting coated article 11 as schematically shown consists of thesubstrate 3, composite barrier layer 9, and a layer 13 formed of RACprecursors.

In Step D of the preparation process, the RAC precursor layer isconverted into an electrically conductive crystalline RAC layer. Step Dentails one or more heating steps in which volatilizable ligandscontained within the RAC precursor are removed from the layer 13,oxidation of the rare earth, alkaline earth, and copper metals occurs,and crystallization of the resulting RAC layer occurs. As schematicallyshown, product circuit element 15 consists of the substrate 3, compositebarrier layer 9, and conductive RAC layer 17. Depending upon specificchoices of materials and preparation techniques, the article 17 canexhibit a high superconducting transition temperature, herein employedto designate a T_(c) of greater than 30° C.

A preferred process for preparing thin (<5 μm) film circuit elementsaccording to this invention once a substrate having a composite barrierlayer has been produced can be appreciated by reference to FIG. 2. InStep C1 of the preparation process, onto a composite barrier cladsubstrate is coated a solution consisting essentially of a volatilizablefilm forming solvent and metal-ligand compounds of each of rare earth,alkaline earth, and copper containing at least one thermallyvolatilizable ligand. The resulting coated article 11a as schematicallyshown consists of barrier clad substrate 7 and a layer 13a formed by RACprecursors (metal-ligand compounds) and film forming solvent.

In Step D1 article 11a is heated to a temperature sufficient tovolatilize the ligands and the film forming solvent. The element 15aresulting consists of barrier clad substrate 7 and amorphous RAC layer17a. In its amorphous form the RAC coating exhibits relatively lowlevels of electrical conductivity.

To convert the amorphous RAC layer to a more highly conductive form itis necessary to induce crystallization of the RAC layer. In Step D2 thearticle 15a is heated to a temperature sufficient to convert theamorphous RAC layer to a more electrically conductive crystalline form.In article 15b the RAC layer 17b on barrier clad substrate 7 iscrystalline.

Crystallization of the RAC layer occurs in two stages--crystalnucleation and crystal growth. It is in some instances preferred toachieve crystal nucleation at a somewhat different temperature than isemployed for crystal growth. Microscopic examination of articles at anearly stage of crystallization reveals crystal nuclei surrounded by atleast one other RAC phase. Further heating of the RAC layer at thetemperature of nucleation or, preferably, at a somewhat highertemperature increases the size of the crystal nuclei at the expense ofthe surrounding RAC phase or phases until facets of adjacent crystalsare grown into electrically conductive juxtaposition.

According to accepted percolation theory, for a layer consisting ofconducting spheres randomly located in a surrounding nonconductingmedium the spheres must account for at least 45 percent by volume of thelayer for satisfactory electrical conductivity to be realized. Ifconducting particles of other geometric forms, particularly elongatedforms, are substituted for the spheres, the conducting particles canaccount for less of the layer volume while still realizing satisfactorylayer electrical conductivity. Similarly, electrical conductivity can berealized with a lesser proportion of conducting particles when thesurrouding medium is also conductive. Thus, all layers containing atleast 45 percent by volume electrically conductive particles are bytheory electrically conductive.

Although satisfactory electrical conductivity can be realized with alesser volume of the crystalline phase, it is generally contemplatedthat in the crystallized RAC layer the crystalline phase will accountfor at least 45 percent by volume and preferably 70 percent by volume ofthe total RAC layer. From microscopic examination of highly crystallineRAC layers exhibiting high levels of electrical conductivity it has beenobserved that layers can be formed in which little, if any, of the RACphase surrounding the crystal nuclei remains. In other words greaterthan 90 percent (and in many instances greater than 99 percent) byvolume of the RAC layer is accounted for by the desired crystallinephase.

To achieve crystallization the RAC layer can be heated to any convenienttemperature level. While the composite barrier allows heating to highercrystallization temperatures than would otherwise be acceptable, it isgenerally preferred that the RAC layer be heated no higher than isrequired for satisfactory crystallization. Heating to achievecrystallization can, for example, be limited to temperatures below themelting point of the RAC composition forming the layer. From microscopicexamination of RAC layers optimum heating times can be selected formaximizing both the proportion of the RAC layer accounted for by thecrystalline phase and the desired configuration of the crystalsproduced, thereby maximizing electrical conductivity.

Step D3 entails controlled cooling of the RAC layer from itscrystallization temperature. By slowing the rate of cooling of thecrystalline RAC layer imperfections in the crystal lattices can bereduced and electrical conductivity, which is favored with increasingorder in the crystal structure, is increased. Cooling rates of 25° C.per minute or less are contemplated until the crystalline RAC layerreaches a temperature of at least 500° C. or, preferably, 200° C. Belowthese temperatures the lattice is sufficiently rigid that the desiredcrystal structure is well established. The article 15c produced isformed of the annealed crystalline RAC layer 17c on barrier cladsubstrate 7.

While the article 15c exhibits high levels of electrical conductivity,in some instances further heating of the article 15c in an oxygenenriched atmosphere has been observed to increase electricalconductivity further. In addition to oxygen supplied from the ligandsthe oxygen forming the crystalline RAC layer is obtained from theambient atmosphere, typically air. It is believed that in someinstances, depending upon the crystal structure being produced, ambientair does not provide the proportion of oxygen needed to satisfy entirelythe available crystal lattice sites.

Therefore, optional Step D4 entails heating the article 15c in an oxygenenriched atmosphere, preferably pure oxygen. The object is toequilibrate the RAC crystalline layer with the oxygen enrichedatmosphere, thereby introducing sufficient oxygen into the crystallattice structure. Temperatures for oxygen enrichment of the crystallineRAC layer are above the minimum 200° C. annealing temperatures employedin Step D3 described above. To be effective in introducing oxygen intothe crystal lattice temperatures above those at which the latticebecomes rigid are necessary. The duration and temperature of heating areinterrelated, with higher temperatures allowing shorter oxygenenrichment times to be employed.

In preparing RAC layers shown to be benefitted by oxygen enrichment ofthe ambient atmosphere Step D4 can be consolidated with either or bothof Steps D2 and D3. Oxygen enrichment is particularly compatible withStep D3, allowing annealing out of crystal lattice defects andcorrection of crystal lattice oxygen deficiencies to proceedconcurrently.

The final electrically conductive article 15d is comprised of acrystalline, electrically conductive RAC layer 17d on barrier cladsubstrate 7.

The process described for preparing electrically conductive articleshaving RAC layers offers several distinct advantages. One of the mostsignificant advantages is that the electrically conductive RAC layer isprotected from direct contact with the substrate throughout the process.This allows, but does not require, higher temperatures to be employed inproducing the conductive RAC layer. Further, thinner RAC layers havingacceptable electrical conduction properties can be realized. In manyinstances the presence of the composite barrier allows superconductiveand particularly high transition temperature superconductive RAC layercharacteristics to be obtained which would be difficult or impossible torealize in the absence of the composite barrier.

Another significant advantage of the process described above is that theproportions of rare earth, alkaline earth, and copper elements in thefinal RAC layer 17d exactly correspond to those present in the RACprecursor layer 13a. In other words, the final proportion of rare earth,alkaline earth, and copper elements is determined merely by mixing inthe desired proportions in the film forming solvent the metal-ligandcompounds employed as starting materials. This avoids what can betedious and extended trial and error adjustments of proportions requiredby commonly employed metal oxide deposition techniques, such assputtering and vacuum vapor deposition. Further, the present processdoes not require any reduction of atmospheric pressures, and thus noequipment for producing either high or low vacuum.

A further significant advantage of the process of this invention is thatit can be applied to the fabrication of electrically conductive articlesof varied geometry, particularly those geometrical forms most commonlyencountered in silicon semiconductor devices, including discretedevices, hybrid circuit devices, and integrated circuit devices. In suchcircuits limited, if any, flexibility of the electrical conductor isrequired, but an ability to define areally--i.e., pattern, theelectrical conductor with a high degree of precision is in manyinstances of the utmost importance. The present invention is compatiblewith precise patterning of the electrical conductor on a substratesurface.

Patterning of an electrical conductor according to this invention isillustrated by reference to FIG. 3. Barrier clad substrate 7 is coatedon its upper planar surface with a uniform RAC precursor layer 13a asdescribed above in connection with process Step C1 to form initialcoated article 11a. Process Step D1, described above, is performed onarticle 11a to produce article 15a, described above, comprised ofamorphous RAC layer 17a and barrier clad substrate 7.

The amorphous RAC layer lends itself to precise pattern definition andproduces results generally superior to those achieved by patterning theRAC precursor layer from which it is formed or the crystalline RAC layerwhich is produced by further processing. The RAC precursor layer isoften liquid before performing process Step D1 and is in all instancessofter and more easily damaged in handling than the amorphous RAC layer.The crystalline RAC layer cannot be etched with the same boundaryprecision as the amorphous RAC layer, since etch rates vary from pointto point based on local variations in the crystal faces and boundariespresented to the etchant. Patterning of either the RAC precursor layeror the crystalline RAC layer is specifically recognized as a viablealternative to patterning the amorphous RAC layer for applicationspermitting more tolerance of conductor dimensions. For example, screenprinting the RAC precursor layer on a substrate to form a printedcircuit is specifically contemplated.

While the amorphous RAC layer can be patterned employing anyconventional approach for patterning metal oxides, for more precise edgedefinitions the preferred approach is to photopattern the amorphous RAClayer employing any of the photoresist compositions conventionallyemployed for the precise definition of printed circuit or integratedcircuit conductive layers. In a preferred form of the process, a uniformphotoresist layer 23 is applied to the amorphous RAC layer 17a asindicated by process Step D5. The photoresist layer can be formed byapplying a liquid photoresist composition to the amorphous RAC layer,spinning the substrate to insure uniformity of the coating, and dryingthe photoresist. Another approach is to laminate a preformed photoresistlayer supported on a transparent film to the amorphous RAC layer.

The photoresist layer is then imagewise exposed to radiation, usuallythrough a mask. The photoresist can then be removed selectively as afunction of exposure by development. Positive working photoresists areremoved on development from areas which are exposed to imaging radiationwhile negative working photoresists are removed only in areas which arenot exposed to imaging radiation. Exposure and development are indicatedby process Step D6. Following this step patterned photoresist layer 23ais left on a portion or portions of the amorphous RAC layer 17a.Although the patterned residual photoresist layer is for convenienceshown of a simple geometrical form, it is appreciated that in practicethe patterned photoresist can take any of a wide variety of geometricalforms, including intricate and thin line width patterns, with linewidths ranging into the sub-micrometer range.

Following patterning of the photoresist layer, portions of the RAC layerwhich are not protected by the photoresist can be selectively removed byetching, as indicated by process Step D7. This converts the amorphousRAC layer 17a to a patterned RAC layer 17e confined to areascorresponding to that of the photoresist. Note that in the process ofetching the barrier clad substrate may be modified by removal of thecomposite barrier in unprotected areas to produce a modified barrierclad substrate 7a. Whether or not the unprotected composite barrier isremoved will depend, of course, on the specific etchant employed.However, it is important to note that there is no requirement that theetchant be selective to the amorphous RAC layer as opposed to thebarrier material.

Following patterning of the amorphous RAC layer the patternedphotoresist is removed, as indicated by process Step D8. The finalarticle, shown in FIG. 4 as consisting of the partially barrier cladsubstrate 7a and patterned amorphous RAC layer 17e, is then furtherprocessed as indicated in FIG. 2, picking up with process Step D2. Thecrystalline RAC layer formed in the final product conforms to thepatterned amorphous RAC layer.

In the process of preparing a patterned article described above it isnoted that once an article is formed having an amorphous RAC layer on asubstrate it can be patterned to serve any of a wide variety of circuitapplications, depending upon the circuit pattern chosen. It is thereforerecognized that instead of or as an alternative to offering patternedarticles for sale a manufacturer can instead elect to sell articles withunpatterned amorphous RAC layers on a barrier clad substrate, with orwithout an unpatterned photoresist layer, to subsequent fabricators. Itwill often be convenient in this instance to locate a removable layer orfilm over the amorphous RAC layer for its protection prior to furtherfabrication. The subsequent fabricator can undertake the patternedexposure and further processing required to produce a finishedelectrical circuit element.

To crystallize a RAC layer and to perform the optional, but preferredannealing and oxygen enrichment steps both the substrate and RAC layerare heated uniformly. This can be done employing any conventional oven.In some instances, however, either to protect the substrate from risingto the peak temperatures encountered by the RAC layer or simply to avoidthe investment in an oven by fabricator, it is contemplated that the RAClayer will be selectively heated. This can be accomplished by employinga radiant heat source, such as a lamp--e.g., a quartz lamp. Lamps ofthis type are commercially available for achieving rapid thermalannealing of various conventional layers and can be readily applied tothe practice of the invention. These lamps rapidly transmit high levelsof electromagnetic energy to the RAC layer, allowing it to be brought toits crystallization temperature without placing the substrate in anoven.

A diverse approach for producing patterned electrical conductors can bepracticed by employing article 15a comprised of the uniform amorphousRAC layer 17a and barrier clad substrate 7 as a starting element.Instead of patterning the amorphous RAC layer followed bycrystallization of the remaining portions of the layer, the amorphousRAC layer is imagewise addressed to produce crystallization selectivelyonly in areas intended to be rendered electrically conductive. Forexample, by addressing the amorphous RAC layer with a laser, areasdirectly impinged by the laser beam can be selectively crystallized toan electrically conductive form, leaving the remaining amorphous areasunaffected. To define the conductive pattern generated it is onlynecessary to control the path of the laser beam.

Where a manufacturer chooses to sell an article consisting of a uniformamorphous RAC layer on a barrier clad substrate, this approach topatterning can be more attractive than the uniform heating processesdescribed above, since no oven is required to reach the temperaturestypically required for crystallization. The fabricator choosing laserpatterning may, in fact, require no other heating equipment. Thus, avery simple approach to forming a crystalline RAC pattern is available.

It is, of course, recognized that additional heating for purposes ofannealing or oxygen saturation can be undertaken, following lamp orlaser crystallization, by heating in any desired manner. One approach isto heat at least amorphous layer 17a of the article 15a to a temperatureabove its minimum annealing temperature and then laser address theheated article. This facilitates annealing and oxygen enrichment withoutrequiring heating the entire article uniformly to the significantlyhigher levels otherwise required for crystal nucleation and growth.

Another variation on the laser patterning approach is to follow thelaser responsible for crystallization with one or more passes from alower intensity laser beam to retard the rate of cooling and therebyenhance annealing. For example, a laser beam can be swept across an areaof the substrate surface to produce crystallization and then reduced inintensity or defocused and swept back across the same area to facilitateannealing. By defocusing the laser beam on subsequent passes over thesame area the laser energy is spread over a larger area so that themaximum effective temperature levels achieved are reduced. The advantageof employing one laser for multiple passes is that alignments of laserbeam paths are more easily realized. Additionally or alternatively, therapidity with which the laser is swept across the exposed area can beadjusted to control the temperature to which it heats the RAC layer.Other laser scanning variations are, of course, possible.

Both lamp heating and laser scanning allow a broader range of substratematerials to be considered, particularly those which, though capable ofwithstanding ligand and solvent volatilization temperatures, aresusceptible to degradation at crystallization temperatures. By choosingwavelengths in spectral regions to which the amorphous RAC layer isopaque or at least highly absorbing, direct radiant heating of thesubstrate can be reduced or eliminated. In this instance the bulk of theradiation is intercepted in the RAC layer before it reaches thecomposite barrier. The silicon substrate is also protected from directradiant heating by the composite barrier. By proper choice of radiantenergy wavelengths the composite barrier can reflect a high proportionof total radiant energy received.

To avoid coating imperfections in the thin film process described abovethe thickness of an amorphous RAC layer produced in a single processsequence is maintained at 1 μm or less, preferably 0.6 μm or less, andoptimally 0.4 μm or less, a single process sequence being understood toconstitute the steps described above for forming an amorphous RAC layer.By repeating the process sequence one or more times an amorphous RAClayer of any desired thickness can be built up.

In the process of fabrication described above the formation of thedesired RAC layer begins with the formation of a RAC precursor layer. Toform the precursor layer a solution of a film forming solvent, a rareearth metal compound, an alkaline earth metal compound, and a coppercompound is prepared. Each of the rare earth, alkaline earth, and coppercompounds consists of metal ion and one or more volatilizable ligands.By "volatilizable" it is meant that the ligand or its component elementsother than oxygen can be removed from the substrate surface attemperatures below the crystallization temperature of the RAC layer. Inmany instances organic ligands breakdown to inorganic residues, such ascarbonates, at relatively low temperatures, with higher temperaturebeing required to remove residual carbon. A ligand oxygen atom bondeddirectly to a metal is often retained with the metal in the RAC layer,although other ligand oxygen atoms are generally removed. At least 95percent of the ligands and their component atoms other than oxygen arepreferably outgassed at temperatures of less than 600° C. On the otherhand, to avoid loss of materials before or during initial coating of themetal-ligand compounds, it is preferred that the ligands exhibitlimited, if any, volatility at ambient temperatures. Metal-ligandcompounds having any significant volatility below their decompositiontemperature are preferably avoided.

Metalorganic compounds, such as metal alkyls, alkoxides, β-diketonederivatives, and metal salts of organic acids--e.g., carboxylic acids,constitute preferred metal-ligand compounds for preparing RAC precursorcoatings. The number of carbon atoms in the organic ligand can vary overa wide range, but is typically limited to less than 30 carbon atoms toavoid unnecessarily reducing the proportion of metal ions present.Carboxylate ligands are particularly advantageous in promotingmetal-ligand solubility. While very simple organic ligands, such asoxalate and acetate ligands, can be employed in one or moremetal-ligands compounds, depending upon the film forming solvent andother metal-ligand compound choices, it is generally preferred to chooseorganic ligands containing at least 4 carbon atoms. The reason for thisis to avoid crystallization of the metal-ligand compound and to improvesolubility. When heating is begun to remove the film forming solvent andligands, the solvent usually readily evaporates at temperatures wellbelow those required to remove the ligands. This results in leaving themetal-ligand compounds on the substrate surface. When the ligands havefew carbon atoms or, in some instances, linear carbon atom chains,crystallization of the metal-ligand compounds occurs. In extreme casescrystallization is observed at room temperatures. This works against themolecular level uniformity of rare earth, alkaline earth, and copperions sought by solution coating. Choosing organic ligands exhibiting 4or more carbon atoms, preferably at least 6 carbon atoms, and,preferably, ligands containing branched carbon atom chains, reducesmolecular spatial symmetries sufficiently to avoid crystallization.Optimally organic ligands contain from about 6 to 20 carbon atoms.

Instead of increasing the molecular bulk or modifying the chainconfiguration of organic ligands to avoid any propensity towardmetalorganic compound crystallization on solvent removal, anothertechnique which can be employed is to incorporate in the film formingsolvent a separate compound to act as a film promoting agent, such as ahigher molecular weight branched chain organic compound. This can, forexample, take the form of a branched chain hydrocarbon or substitutedhydrocarbon, such as a terpene having from about 10 to 30 carbon atoms.

The film forming solvents can be chosen from a wide range ofvolatilizable liquids. The primary function of the solvent is to providea liquid phase permitting molecular level intermixing of themetalorganic compounds chosen. The liquid is also chosen for its abilityto cover the substrate uniformly. Thus, an optimum film forming solventselection is in part determined by the substrate chosen. Generally moredesirable film forming properties are observed with more viscoussolvents and those which more readily wet the substrate alone, or withan incorporated wetting agent, such as a surfactant, present.

It is appreciated that a wide variety of ligands, film promoting agents,and film forming solvents are available and can be collectively presentin a virtually limitless array of composition choices.

Exemplary preferred organic ligands for metal organic compounds includemetal 2-ethylhexanoates, naphthenates, neodecanoates, butoxides,isopropoxides, rosinates (e.g., abietates), cyclohexanebutyrates, andacetylacetonates, where the metal can be any of the rare earth, alkalineearth, or copper elements to be incorporated in the RAC layer. Exemplarypreferred film forming agents include 2-ethylhexanoic acid, rosin (e.g.,abietic acid), ethyl lactate, 2-ethoxyethyl acetate, and pinene.Exemplary preferred film forming solvents include toluene,2-ethylhexanoic acid, n-butyl acetate, ethyl lactate, propanol, pinene,and mineral spirits.

As previously noted, the metal-ligand compounds are incorporated in thefilm forming solvent in the proportion desired in the final crystallineRAC layer. The rare earth, alkaline earth, and copper can each bereacted with the same ligand forming compound or with different-ligandforming compounds. The metal-ligand compounds can be incorporated in thefilm forming solvent in any convenient concentration up to theirsaturation limit at ambient temperature. Generally a concentration ischosen which provides the desired crystalline RAC layer thickness forthe process sequence. Where the geometry of the substrate permits,uniformity and thickness of the metal-ligand coating can be controlledby spinning the substrate after coating around an axis normal to thesurface of the substrate which has been coated. A significant advantageof spin coating is that the thickness of the coating at the conclusionof spinning is determined by the contact angle and viscosity of thecoating composition and the rate and time of spinning, all of which canbe precisely controlled. Differences in the amount of the coatingcomposition applied to the substrate are not reflected in the thicknessof the final coating. Centrifugal forces generated by spinning causeexcess material to be rejected peripherally from the article.

The foregoing process of coating RAC precursors in solution isparticularly suited to forming thin films. The term "thin film" isemployed to indicate films having thicknesses of less than 5 μm, suchfilms most typically having thicknesses of less than 1 μm. The term"thick film" is employed in its art recognized usage to indicate filmshaving thicknesses in excess of 5 μm.

A preferred process for producing thick film electrically conductive RAClayers on barrier clad substrates can be appreciated by reference to theschematic diagram shown in FIG. 4. In Step C2 a composition containingparticles of metal-ligand compounds is obtained. Each particle containsrare earth, alkaline earth, and copper atoms in the same ratio desiredin the final RAC containing conductive layer. Further, the atoms areintimately intermixed so that the composition of each particle ispreferably essentially uniform. Associated with the metal atoms andcompleting the compounds are volatilizable ligands, which can be allalike or chosen from among different ligands.

The particles can be of any size convenient for coating. The particlescan exhibit a mean diameter up to the thickness of the coating to beformed, but more uniform films are realized when the mean particlediameters are relatively small in relation to the thickness of the filmto be formed. The particles are preferably less than about 2 μm in meandiameter, optimally less than 1 μm in mean diameter. The minimum meandiameter of the particles is limited only by synthetic convenience.

A preferred technique for producing metal-ligand compound particles isto dissolve the rare earth, alkaline earth, and copper metal ligandcompounds in a mutual solvent and then to spray the solution through anatomizing nozzle into a gaseous atmosphere. The solvent is chosen to beevaporative in the gaseous atmosphere. Thus, the individual particlesare dispersed in the gaseous atmosphere as liquid particles andeventually come to rest at a collection site as either entirely solidparticles or particles in which the proportion of solvent has beensufficiently reduced that each of the metal-ligand compounds present hasprecipitated to a solid form. In the latter instance the particles byreason of the residual solvent, now no longer acting as a solvent, butonly as a continuous dispersing phase, form a paste. The pasteconstitutes a highly convenient coating vehicle. When the particles arecollected in a friable form with all or substantially all of theinitially present solvent removed, it is recognized that a paste canstill be formed, if desired, by adding to the particles a small amountof a liquid to promote particle cohesion--i.e., to constitute a paste.

Only a very small amount of liquid is required to promote particlecohesion and thereby form a paste. Typically the liquid constitutes lessthan 20 percent of the total composition weight and preferably less 15percent of the total composition weight. While optimum pasteconsistencies can vary depending upon the selection of processes forcoating the paste, it is generally contemplated that the paste viscositywill be in the range of from 5×10⁴ to 3×10⁶ centipoise, preferably from1×10⁵ to 2.5×10⁶ centipoise.

While atomization and drying can be undertaken in air at roomtemperatures, it is recognized that any gaseous medium which does notdetrimentally react with the metal-ligand compounds can be employed.Further, the temperature of the liquid forming the particles or,preferably, the gaseous medium can be increased to accelerate thesolvent evaporation rate, provided only that such elevated temperaturesin all instance be maintained below the thermal decompositiontemperatures of the metal-ligand compounds.

The advantage of solidifying the metal-ligand compounds while they aretrapped within discrete particles is that bulk separations of the rareearth, alkaline earth, and copper are prevented. The particlepreparation approach offers distinct advantages over simply evaporatingbulk solutions to dryness in that each particle produced by the processof this invention contains the desired ratio of rare earth, alkalineearth, and copper elements. This produces a solid particle coatingcomposition of microscale uniformity.

In Step C3 of the preparation process, onto a substrate are coated themetal-ligand compound particles, preferably combined with a carrierliquid to form a coatable paste or slurry. The resulting coated article11b as schematically shown consists of barrier clad substrate 7 and alayer 13b formed by RAC precursors (metal-ligand compounds) and filmforming solvent. Although the layer 13b is shown coextensive with thebarrier clad substrate 7, it is appreciated that the particles are wellsuited, particularly when coated in the form of a paste or slurry, tobeing laid down in any desired pattern on the barrier clad substrate.The paste can, for example, be deposited by any of a variety ofconventional image defining coating techniques, such as screen orgravure printing. Since thick conductive films are most commonly formedin the art by screen printing, the present invention is highlycompatible with conventional printed circuit preparation processes.

The ligands in the RAC precursor compounds of the thick film processlike those of thin film process form no part of the final article andtherefore can be chosen based solely upon convenience in performing theprocess steps described above. Ligands are chosen for their ability toform solutions in which rare earth, alkaline earth, and copper combinedwith the ligands are each soluble in the desired proportions and to bevolatilizable during heating to form the intermediate RAC layer.Inorganic ligands, such as nitrate, sulfate, and halide ligands, areillustrative of preferred ligands satisfying the criteria set forthabove. Nitrate, bromide, and chloride ligands are particularlypreferred. In general the ligands are chosen so that each of the rareearth, alkaline earth, and copper ligand compounds exhibit approximatelysimilar solubility characteristics.

Any evaporative solvent for the metal-ligand compounds can be employedfor particle fabrication. Again, the solvent forms no part of the finalarticle. Polar solvents, such as water or alcohols (e.g., methanol,ethanol, propanol, etc.), are particularly suited for use withmetal-ligand compounds containing the inorganic ligands noted above.

Where a paste is coated, the paste contains either a small residualportion of the original solvent for the metal-ligand compounds or adifferent liquid to promote cohesion. The liquid fraction of the pastemust be volatilizable. The evaporative solvents noted above all satisfythis criteria.

The paste, apart from the metal-ligand particles, can be identical incomposition to conventional inks employed in screen printing. Screenprinting inks normally contain an active ingredient (in this instancesupplied by the metal-ligand particles), binders to promote substrateadhesion (such as glass frit or crystalline oxide powder), screeningagents used to enhance the rheological properties of the ink--usually ahigher molecular weight polymer, such as poly(vinyl alcohol) orpoly(ethylene glycol), and a liquid, most commonly water or an alcohol.It is a particular advantage of this invention that the metal-ligandparticles and liquid together provide excellent rheological and adhesionproperties without the necessity of incorporating other screen printingink ingredients.

Heating step D can be performed as described above can then beundertaken to produce final article 15e consisting of thick film RAClayer 17f on barrier clad substrate 7 as described above in connectionwith FIG. 1. The overall heating step D can include the same sequence ofsteps D1, D2, D3, and D4 described above in connection with FIG. 2.

In addition to all of the advantages described above for the preferredthin film forming process, a particular advantage of the thick filmprocess is that it readily lends itself to the formation of electricalconductor patterns on limited portions of substantially planar substratesurfaces without resorting to uniform coatings followed by etching todefine a pattern. This is a convenience which assumes an added level ofimportance in producing thick film conductors. Thus, the present processis readily applied to the fabrication of printed and hybrid circuits.The thick film process can also be employed to form RAC layers of lessthan 5 μm in thickness--that is, it is capable of forming either thickor thin film electrical circuit elements.

To achieve articles according to this invention which are not onlyelectrically conductive, but also exhibit high T_(c) levels, therebyrendering them attractive for high conductivity (e.g., superconducting)electrical applications, RAC layers are produced in specific crystallineforms. One specifically preferred class of high T_(c) articles accordingto this invention are those in which the crystalline RAC layer consistsof greater than 45 percent by volume of a rare earth alkaline earthcopper oxide which is in a tetragonal K₂ NiF₄ crystalline phase. The K₂NiF₄ crystalline phase preferably constitutes at least 70 percent andoptimally at least 90 percent by volume of the RAC layer.

One rare earth alkaline earth copper oxide exhibiting this crystallinephase satisfies the metal ratio:

    L.sub.2-x :M.sub.x :Cu                                     (I)

where

L is lanthanide,

M is alkaline earth metal, and

x is 0.05 to 0.30.

Among the preferred lanthanides, indicated above, lanthanum has beenparticularly investigated and found to have desirable properties.Preferred alkaline earth metals are barium and strontium. Optimumresults have been observed when x is 0.15 to 0.20.

Thus, in specifically preferred forms of the invention LBC or LSC layersexhibiting a tetragonal K₂ NiF₄ crystalline phase are present andcapable of serving high conductivity applications, including thoserequiring high T_(c) levels and those requiring superconductivity attemperatures in excess of 10° K. Specific LBC layers in the tetragonalK₂ NiF₄ crystalline phase have been observed to have T_(c) levels inexcess of 40° K.

Another specifically preferred class of high T_(c) articles according tothis invention are those in which the crystalline RAC layer consists ofgreater than 45 percent by volume of a rare earth alkaline earth copperoxide which an R₁ A₂ C₃ crystalline phase, believed to be anorthorhombic Pmm2 or orthorhombically distorted perovskite crystalphase. This phase preferably constitutes at least 70 percent by volumeof the RAC layer.

A preferred rare earth alkaline earth copper oxide exhibiting thiscrystalline phase satisfies the metal ratio:

    Y:M.sub.2 :Cu.sub.3                                        (II)

where

M is barium, optionally in combination with one or both of strontium andcalcium.

Although the R₁ A₂ C₃ crystalline phase by its crystal latticerequirements permits only a specific ratio of metals to be present, inpractice differing ratios of yttrium, rare earth, and copper arepermissible. The metal in excess of that required for the R₁ A₂ C₃crystalline phase is excluded from that phase, but remains in the YAClayer.

Processing temperatures employed in forming the amorphous RAC-layers andin subsequently converting the amorphous layers to crystalline layerscan vary significantly, depending upon the specific RAC composition andcrystal form under consideration. Crystallization is in all instancesachieved below the melting point of the RAC composition. Melting pointsfor RAC compositions vary, but are typically well above 1000° C. TypicalRAC crystallization temperatures are in the range of from about 900° to1100° C. Where crystal nucleation and growth are undertaken in separatesteps, nucleation is preferably undertaken at a somewhat lowertemperature than crystal growth.

In some instances X-ray diffraction has revealed the presence ofmicrocrystals in the amorphous RAC layer, although limited to minoramounts. While crystallization of the metal-ligand compounds, whichtends to separate the metals into different phases, is generallyavoided, crystallization which occurs during or immediately followingligand volatilization is not objectionable, since metals absent theirligands are free to form mixed metal oxides.

A preferred technique for producing a high T_(c) coating employing anamorphous layer of the LAC composition metal ratio I above, particularlyan LBC or LSC composition, is to heat the amorphous layer on thesubstrate to a temperature of about 925° to 975° C. to achieve crystalnucleation. Crystal growth is then undertaken at a temperature of about975° to 1050° C. Following conversion of the LAC layer to the tetragonalK₂ NiF₄ crystalline phase, it is cooled slowly at rate of of 25° C. orless per minute until it reaches a temperature of 550° to 450° C. TheLAC layer is then held at this temperature or reheated to thistemperature in the presence of an oxygen atmosphere until oxygenequilibration is substantially complete, typically about 20 to 120minutes.

A preferred technique for producing a high T_(c) coating employing anamorphous layer of the YAC composition satisfying metal ratio II above,particularly YBC, is to heat the amorphous layer on the substrate to atemperature of a temperature greater than 900° C., but less than 950°C., optimally 920° to 930° C. Following conversion of the LAC layer tothe R₁ A₂ C₃ crystalline phase, it is cooled slowly at rate of of 25° C.or less per minute until it reaches a temperature of 750° to 400° C. TheYAC layer is then held at this temperature or reheated to thistemperature following cooling in the presence of an oxygen atmosphereuntil oxygen equilibration is substantially complete, typically about 20to 120 minutes.

EXAMPLES

Details of the preparation and performance of articles according to thisinvention are illustrated by the following examples:

EXAMPLE 1

A silica (SiO₂) layer 5000 Å thick was formed on a monocrystallinesilicon substrate by thermal oxidation. A zirconium metal film 1500 Å inthickness was then evaporated on the silica layer and subsequentlyannealed in vacuum at 850° C. for 30 minutes to produce a barrier layertriad of the following configuration:

    ______________________________________                                        Zirconia                                                                      Zirconium Silicide                                                            Silica                                                                        Silicon Substrate                                                             ______________________________________                                    

A high transition temperature superconductive YBC layer was depositedonto the barrier layer using the following technique:

A yttrium containing solution was prepared by mixing and reactingyttrium acetate with a stoichiometric excess of 2-ethylhexanoic acid toproduce yttrium tri(2-ethylhexanoate) in 2-ethylhexanoic acid. Theresulting solution contained 7.01 percent by weight yttrium, based ontotal weight.

A copper containing solution was prepared by mixing and reacting copperacetate with a stoichiometric excess of 2-ethylhexanoic acid to formcopper di(2-ethylhexanoate). This solution contained 6.36 percent byweight copper, based on total weight.

A 0.81 g sample of the yttrium containing solution and a 1.92 gramsample of the copper containing solution were mixed followed by theaddition of 0.66 gram of barium di(cyclohexanebutyrate), 0.4 gram oftoluene, and 0.7 gram of rosin. Heat was applied until all ingredientshad entered solution, thereby forming a YBC precursor solution.

The YBC precursor solution was deposited onto the composite barrier byspin coating at 5000 rpm for 20 seconds. The coated substrate had asmooth and uniform appearance with no imperfections being noted onvisual inspection, indicating favorable rheological properties.

The YBC precursor coated composite barrier and silicon substrate wereheated on a hot plate to 550° C. to eliminate organic ligands from thecoating. The film forming process was repeated 3 times.

The amorphous RAC layer exhibited a 1:2:3 atomic ratio of Y:Ba:Cu and athickness of about 1 μm. The amorphous YBC layer was heated to 900° C.for 3 minutes and allowed to cool at a rate of less than 25° C. perminute.

The resulting films were measured by Rutherford backscatteringspectrometry (RBS), scanning electron microscopy (SEM), energydispersive spectrometry (EDS), X-ray diffraction, and scanning Augerprofiling techniques. In some instances silver contact pads were formedon the sample by painting a silver silver metal-organic solution andheating to about 450° C. in oxygen. Four point probe techniques wereemployed to measure the sheet resistance versus temperature.

RBS measurements did not reveal significant interactions between therare earth alkaline earth copper oxide film and the silicon substrate.X-ray diffraction analysis indicated the formation of a well-definedorthorhombic perovskite YBa₂ Cu₃ O_(7-y) structure. Scanning Augerprofiling measurements showed that the amount of silicon and zirconiumin the YBa₂ Cu₃ O_(7-y) was below the sensitivity level of thetechnique. It should be mentioned that these characteristics were alsoobserved when the thickness of the YBa₂ Cu₃ O_(7-y) oxide film asreduced to 2500 Å. When the sheet resistance of the oxide film wasmeasured as a function of temperature, a relatively sharp drop of theelectrical resistivity was observed at temperatures of approximately 80°K.

EXAMPLE 2

A silicon substrate with a composite barrier was prepared as describedin Example 1. On different samples of the barrier clad substrate werevacuum vapor deposited silver layers with thicknesses ranging from 500to 2000 Å. A crystalline rare earth alkaline earth copper oxide coatingwas then formed on the silver layers similarly as described in Example1.

The crysalline YBa₂ Cu₃ O_(7-y) films possesed the perovskite structurewith a strong orientation dependence. SEM revealed a polycrystallinestructure with grain sizes several μm in diameter. With a 500 Å silverlayer the YBC oxide film showed a room temperature resistivity of about5×10⁻³ ohm-cm, which was about 1 order of magnitude lower than thatobserved with the silver layer omitted. Low temperature resistivitymeasurements by a 4-point probe indicated a superconducting transitionat approximately 90° K. Further, superconductivity was observed to occurat higher temperatures than with the silver layer absent.

EXAMPLE 3

The procedure of Example 1 was repeated, but with the composite barrierbeing omitted.

It was found by RBS that copper was completely depleted from the surfacecrystalline oxide layer and had diffused into the silicon substrate. Asa result the film showed no traces of a perovskite R₁ A₂ C₃ crystalstructure. The sheet resistance of the layer was so high as to be almostunmeasurable.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A process of producing on a silicon substrate abarrier layer triad interposed between the conductive layer and thesilicon substrate comprisingforming a silica layer of at least 2000 Å inthickness on the silicon substrate, depositing on the silica at leastone Group 4 heavy metal to form a layer having a thickness in the rangeof from 1500 to 3000 Å, and heating the layers in the absence of areactive atmosphere sufficiently to permit oxygen migration from thesilica layer, thereby forming a barrier layer triad consisting of afirst triad layer located adjacent the silicon substrate consistingessentially of silica, a third triad layer remote from the siliconsubstrate consisting essentially of at least one Group 4 heavy metaloxide, and a second triad layer interposed between the first and thirdtriad layers consisting essentially of at least one Group 4 heavy metalsilicide.
 2. A process according to claim 1 wherein oxygen migration isachieved by heating to a temperature of at least 700° C.
 3. A processaccording to claim 2 wherein oxygen migration is achieved by heating toa temperature in the range of from 750° to 1000° C.
 4. A processaccording to claim 3 wherein oxygen migration is achieved by heating toa temperature in the range of from 800° to 900° C.
 5. A processaccording to claim 3 wherein heating to achieve oxygen migration extendsover a period of from about 30 to 60 minutes.
 6. A process according toclaim 1 wherein the barrier layer triad is further heated in thepresence of oxygen to convert the second triad layer to a mixture ofsilica and at least one Group 4 heavy metal oxide.
 7. A processaccording to claim 1 wherein silver is coated on the third triad layer.8. A process according to claim 7 wherein silver is coated on the thirdtriad layer in a thickness range of from 500 to 2000 Å.